Cardinal numbers refer to the size of a set. (10 apples in a basket) Ordinal numbers refer to the order of elements in a set (third apple put into the basket) you can rearrange an ordered set and retain the same cardinality (ten apples in the basket) but you'd change the order of the elements (switch the third and sixth apple).

The floor number you're on is an ordinal number. You can rearrange the elements while retaining the cardinality of the set, (the total number of floors does not change) but the order of the set is changed (the third floor is switched with the sixth floor).

Hope that clears up the confusion. Have a nice day.

It depends on the convention whether enumerating starts with the zeroth or first. In programming for example indices commonly start at zero. And the numbering of floors is another example of where starting at zero is quite common.

Cardinal numerals refer to amount (one, two, three) and ordinal refers to a position in a sequence (first, second, third). So your example is ordinal not cardinal.